Outdoor terminations are used for connecting a cable to an electric overhead line. The outdoor terminations usually include a housing and an insulating covering provided with a device for connection to a support pylon. At an inside of the housing, an end portion of the cable is devoid of its outer screening layer and provided with a field control element. The conductor of the cable extends until the end of the insulating covering, to be brought into communication with the overhead line.
The insulating covering commonly consists of a porcelain element performing the function of ensuring insulation between the end portion of the live cable conductor and the earthed supporting structure of the terminal. The insulating covering has a surface extension sufficient to restrain the current passage along the outer surface of the terminal.
An electric cable of the extruded insulating type mainly includes a central conductor consisting for example of a metal cord, made of copper or the like, coated with a semiconductive layer, an insulating layer, a screen electrically earthed, and a protection sheath.
At its entry into the terminal, the cable is devoid of the sheath and screen, and field control elements are used for restraining the electric field gradients at the screen end. Field control element can be either a suitably radiated conductive body, usually referred to as electrode or baffle, commonly made of a semiconductive elastomeric material incorporated in a body of insulating material, the whole being sized so as to keep the electric field gradient within acceptable limits, or a material having a variable dielectric constant and conductivity depending on the electric gradient, or a capacitor system.
A termination for outdoor environments in particular may include a base plate to which the base of a ribbed body made of porcelain is linked, to the upper end of which the cable conductor is connected through appropriate supporting and connecting elements. An earth electrode and a field control cone, of elastomeric material, is forced onto the cable insulating surface within a cylinder of epoxy resin, at its entry into the ribbed body, whereas the free space within the ribbed body is filled with an electrically insulating oil.
The insulating oil within the porcelain covering attempts to eliminate the air subjected to possible ionization where the electric field is higher, which brings about impairment of the terminal integrity. Such a conventional terminal is mounted in an upright position, being linked at the base thereof to a bearing structure. The rigid material of the housing provides the mechanical support of the termination. However, leaking oil filling may cause environmental issues and it can lead to product failure. Moisture ingress may degrade the electrical performance of the oil filling which might cause product failure as well. So-called “dry” terminations avoid any of these problems and furthermore dispense with the manual process of handling liquid oils in the field.
There exist several concepts for dry HV terminations. For example, FIG. 1A shows a conventional termination 200 comprises a thick wall epoxy housing 202, a so-called “PHVX type” plug-in connection 204 (see FIG. 1B) to the inner conductor of a cable 104 and a thin wall silicone shed molding 208. A silicone stress cone 212 with a conductive deflector 206 is provided in the region where the electrically conductive insulation screen 103 of the cable 104 has been removed. The mechanical stability and stiffness needed for the upright or angled mounting position is provided by the rigidity of the electrically insulating housing 202. In order to guarantee a sufficient mechanical stability of the plug-in connection 204, a spring loaded compression ring 210 is provided at the stress cone 212.
A similar concept is illustrated in U.S. Pat. No. 6,677,528 B2. Furthermore, it is known from U.S. Pat. No. 4,757,159 to use a plurality of long rod insulators to stabilize a non-self-supporting termination. A further known product uses a hollow-core insulator and a factory done gel filling instead of liquid silicone oil filling. U.S. Pat. No. 6,333,462 discloses a dry termination using a metal rod with epoxy resin layer around it as the load carrying element. In particular, U.S. Pat. No. 6,333,462 discloses a massive metal rod plus rigid resin overmolding over the entire length of the termination.
European Patent Application No. 0683555 A1 discloses another terminal for a high-voltage electric cable, devoid of insulating oil or deformable fillers, wherein a stiff insulating tubular element is provided with at least one conductive tubular portion. The conductive tubular portion is electrically connected to the upper end portion of the bare conductor and is disposed around the insulating layer of the cable from which the semiconductive screen has been removed. The inner cable conductor extends through the complete length of the termination and is connected with a top plate that is attached to the overhead line. A ribbed coating of elastomeric material is fitted on the tubular element. The stiff tubular element and the conductive portion rest on an insulating housing providing electric field control means at its inside, which means is disposed about the cable starting from the area where the screen has been removed. The tubular element and the conductive portion form a screen and a structure designed to withstand the cable weight and the transverse forces.